Conventional photography is based on a photographic film composed of a flexible backing coated with a silver-based light-sensitive emulsion. In this disclosure, such photographic film will be called conventional photographic film. While photography based on conventional photographic film has been highly refined over the years, it has several problems. First, conventional photographic film-based photography is environmentally objectionable. Conventional photographic films involve noxious chemicals such as silver and require the use of chemical developers whose disposal in an environmentally-acceptable manner is becoming increasingly more costly.
Second, conventional photographic film cannot be reused. Most photographers take several pictures for each picture that is actually kept. This leads to many negatives being thrown away. In addition to the cost of the unused negatives, this practice further aggravates the above-mentioned disposal problems.
Third, unexposed conventional photographic film has a finite shelf life. This increases the cost of photography by requiring refrigerated storage and/or the replacement of film that has passed its usable life.
Fourth, the dynamic range of conventional photographic film is less than adequate for many applications. Even black and white film has a gray scale of only 2.5-3 orders of magnitude. Color film is even more limited. In many applications, the range of intensities that must be recorded greatly exceeds this dynamic range. In such situations, at least some portion of the photograph must be over- or under-exposed.
Finally, correction of artifacts in photographs is difficult in conventional film-based systems. Altering the color of limited regions of a negative is all but impossible. Hence, artifacts such as "red eye" in portraits taken with flash cameras must be handled by using special camera arrangements or by touching-up the prints. The latter approach requires talents not normally possessed by the average photographer.
These disadvantages, together with the increased availability of low-cost computing systems, have generated interest in solid state imaging systems such as CCD cameras and the like. Such cameras store their images on computer-readable media such as flash memories or magnetic disks. Since the image is computer-readable, the image may be altered with the aid of software running on a computer, such as a conventional personal computer. Furthermore, these systems are environmentally superior to conventional photographic film-based system in that noxious chemicals are not required to generate and store pictures. Moreover, the storage medium is reusable. Finally, some solid state systems can have significantly more dynamic range than conventional photographic film.
Unfortunately, solid-state cameras having a resolution equivalent to the resolution available with conventional photographic film are far too expensive for use by the average camera user. These cameras are currently sold for about 100 times the cost of an inexpensive camera using conventional photographic film. Lower cost solid-state cameras are available, but these have inferior resolution to conventional film-based cameras. Solid state cameras also suffer from the disadvantage that they cannot take pictures in quick succession because of the processing time required to compress each picture before the next picture can be taken. Moreover, currently-available solid-state cameras cannot be used with lenses designed for existing photographic film-based camera systems. Finally, a user who is not computer literate may have difficulty in having his or her digital images converted to conventional photographic prints.
Accordingly, there has been some interest in developing new image storage media as an alternative to conventional photographic film. An ideal alternative image storage medium would be a film that could be used in a conventional camera instead of conventional photographic film. Storage phosphors have been known for many years. A potentially useful alternative image storage medium may be made by coating a flexible backing with a thin layer of a storage phosphor. The storage phosphor is doped with two impurities, one of which forms electron traps. Light forming an original image on the surface of the storage phosphor elevates electrons in the storage phosphor from the ground state to the conduction band. The electrons elevated into the conduction band by the incident light are trapped in nearby electron traps. The density distribution of the trapped electrons reflects the light intensity distribution of the light forming the original image. In suitable storage phosphors, the trapped electrons remain trapped by the electron traps for several days.
The latent image stored in the storage phosphor is read out by illuminating the storage phosphor with radiation having a longer wavelength than the light forming the original image. For example, infra-red light may be used to read out a latent image formed by visible light. The infra-red light raises the trapped electrons from the electron traps into the conduction band, whence the electrons fall back to their original states, generating visible light. The intensity of the light generated is proportional to the density distribution of the trapped electrons in the storage phosphor, and, therefore, to the light intensity distribution of the original image.
Reading the latent image out from the storage phosphor bleaches, i.e., partially erases, the latent image. The extent of the bleaching depends on the intensity and duration of the infra-red illumination. Because reading a latent image bleaches it, and because an unread latent image will decay over time, a storage phosphor-based photographic film has only a short-term image storage capability. Latent images stored in a storage phosphor must be transferred to another medium for viewing and for long-term storage.
After the latent image stored in the storage phosphor has been transferred to another storage medium for long-term storage, a prolonged exposure to a high intensity of infra-red light will completely erase the latent image. This leaves the storage phosphor ready to record a new latent image when it is exposed again to visible light. Thus, a photographic system based on a film using storage phosphors has the following advantages. It requires no chemical processing after the latent image has been stored. It uses a reusable storage medium. Finally, it uses a storage medium that has a longer shelf life in its unexposed state than that of conventional unexposed photographic film. However, exploiting the advantages offered by a storage phosphor-based photographic film requires a way of transferring the latent image to another medium quickly, simply and at low cost.
U.S. Pat. No. 2,482,813 awarded to Urbach in 1949 describes a system of photography based on storage phosphors. Urbach discloses a method for long-term image storage by transferring the latent image from the storage phosphor to conventional photographic film by contact printing. The storage phosphor is placed in contact with a conventional photographic film and is flooded with infra-red light. The conventional photographic film records an image of the pattern of visible light emitted by the storage phosphor in response to the infra-red illumination. While this technique works, using the storage phosphor as an intermediary provides no advantage, since the original image could as easily be recorded directly on the conventional photographic film. Moreover, Urbach's process still requires the use of conventional photographic film, with its finite shelf life and it need for developing using noxious chemicals.
U.S. Pat. No. 5,065,023 to Lindmayer describes a storage phosphor-based color photography system. To generate the equivalent of color film, the system disclosed by Lindmayer preferably uses three storage phosphors deposited in layers. Each storage phosphor has two dopants, the first of which is different in the three storage phosphors and determines the color sensitivity of the storage phosphor, i.e., the color of light that will lift an electron into the conduction band of the storage phosphor. The second dopant, which is the same for all three storage phosphors, determines the energy level of the electron trap. The second dopant determines the wavelength of the light used to read out the latent image.
When the three storage phosphor layers are illuminated with infra-red light, each storage phosphor emits light of a different color. The intensity of the light emitted by each storage phosphor depends on the light intensity in the original image in the wavelength range to which the storage phosphor is sensitive. This wavelength range is determined by the first dopant. Usually, the emission spectrum of the storage phosphor are shifted towards longer wavelengths compared with the absorption spectrum. However, the resulting color distortion can be corrected using calibration data and a knowledge of the spectral sensitivities of the dopants.
Lindmayer discloses transferring the latent image to a long-term storage medium by flooding the storage phosphor with infra-red light and recording the resulting visible light emitted by the storage phosphor using an image intensifier and CCD camera. The resulting video signal is subject to computer processing, and the resulting image data may be stored in the computer, and displayed on a CRT or printed using a color printer. The image data can also be stored on magnetic or optical discs for long-term storage. The image data can also be selectively modified to correct errors such as "red eye."
However, by using a CCD camera, Lindmayer's system is subject to the most significant constraint of conventional solid-state cameras, namely, the trade off between resolution and cost. Moreover, Lindmayer's system forms an image of the whole area of the storage phosphor on the image intensifier. This requires a large, complex and expensive lens, since the lens must have a large numerical aperture to maximize the intensity of the image of the latent image that it forms on the image intensifier.
None of the known storage phosphor-based photography systems described above offers a way of reading out the latent image stored in the storage phosphor for long-term storage quickly, simply and at low cost.
The inventor devised a reading device based on a flying-spot scanner for use in his initial experiments in reading latent images stored in storage phosphor-based film. In the reading device, the storage phosphor in which a latent image was stored was mounted on a mechanical x-y stage and an infra-red laser beam was focused on the surface of the latent image to form an illumination spot about 10 .mu.m in diameter. The latent image stored in the storage phosphor was read out one fixel at a time by operating the mechanical x-y stage in steps equal to the size of one fixel to change the position of the illumination spot in the latent image. As will be discussed in greater detail below, a fixel is one picture element of the latent image. The process was repeated until the entire area of the latent image had been scanned in a raster pattern. The visible light emitted by the latent image in response to the infra-red light was collected by a lens and focused on a high-gain photo-multiplier. The picture signal generated by the photo-multiplier for each fixel of the latent image was stored in a computer. The computer was also programmed to apply color correction to the picture signals. The picture signals could then be displayed using the computer's color monitor, and/or printed using a suitable high-resolution color printer. The picture signals could also be stored on the computer's hard disc, or transferred to a storage medium such as a floppy disc, CD-ROM, or flash memory card for long-term storage external to the computer.
The intensity of the visible light emitted by the latent image in response to the infra-red light was so low that the photomultiplier required an integration time of about 200 ms for an adequate signal-to-noise ratio. This reduced the scan rate to about five fixels/s. Since more than six million pixels must be read to provide a spatial resolution comparable with 35 mm photographic film, recording each latent image took about 70 hours. This was far too slow for a commercial product. Moreover, the spatial resolution of this arrangement was less than predicted because the area of the latent image that emitted visible light in response to the illumination spot was greater than the area of the illumination spot itself. The area of visible light emission was greater than the area of the illumination spot due to the particles of the storage phosphor scattering of the infra-red light beam.
Accordingly, an improved apparatus for reading the latent image stored in a storage phosphor is required. The apparatus should scan the latent image in less than two minutes with a spatial resolution comparable with that of conventional photographic film, and should not require the use of a large-diameter, fast lens.